Implementation differences between PreLoader and Loader

// Loader
var ldr:Loader = new Loader();
ldr.contentLoaderInfo.addEventListener(Event.COMPLETE, onLoaded);
ldr.load(new URLRequest("img.jpg"));
addChild(ldr);

// PreLoader
var pldr:PreLoader = new PreLoader();
pldr.addEventListener(Event.COMPLETE, onLoaded);
pldr.load("img.jpg");
addChild(pldr);

function onLoaded(event : Event)
{
  trace("onLoaded");
}